Several methods have been developed to allow the evaluation of neurotoxicity by testing for changes in neurotransmitter-related biochemistry in treated animals. By such approaches, the following factors have been indentified as potential contaminants or modulators of the neurochemical response: (a) Prior handling experience, (b) sex of animal; (c) the vehicle in which the toxic agent is administered, and (d) the age of the animal. Several toxic agents including manganese, acrylamide, chlordecone, and triethyl lead have been found to depress circulating testosterone suggesting the regulation of the level of the hormone is very susceptible to toxic damage. The hypothalamus seems to be more readily affected by toxic agents than other brain areas and this may account for the sensitivity of testosterone levels to change. The increased level of striatal dopamine receptors in acrylamide-treated rats has been shown to be confined to postsynaptic sites.